Hydrofoil craft



March l, 1955 R. R. GILRUTH 2,703,063

HYDROFOIL CAFT Filed Jan. 16, 1951 4 Sheets-Sheet l March l, 1955 R. R. GILRUTH HYDRoFoIL CRAFT 4 Sheets-Sheet 2 Filed Jan. 16, 1951 1N V EN TOR.

Rekm @Mh wy March l, 1955 R. RQ GILRUTH 2,703,063v

HYDRoFoIL CRAFT Filed Jan. 16, 1951 4 Sheets-Sheet 3 FIG. 9. /6/" :,/JZ

INVENTOR ROBERT R. GILRUTH 'f v BY 'fm' JWM v ATTORNEYS March l, 1955 R. R. GILRUTH HYDRoFoIL CRAFT 4 Sheets-Sheet 4 Filed Jan. 16, 1951 R w E H O VT .ww WU ,n ai |I.. G R. m M E B O ,/R /u United States PatentO HYDROFOIL CRAFT Robert R. Glruth, Seaford, Va., assignor to The Hydrofoil Corporation, Washington, D. C., a corporation of Delaware Application January 16, 1951, Serial No. 206,153

24 Claims. (Cl. 114-665) This invention relates to watercraft dynamically supported by hydrofoils. Its primary objective is high hydrodynamic efciency to the end primarily of increasing craft speed from a comparatively low power source of propulsive energy.

In accordance with the preferred embodiment of my invention, the craft is supported by one or more hydrofoils of exceptionally large aspect ratio. The craft is adapted to operate with its entire supporting foil surface constantly submerged and with depth of submergence, rolling and pitching all controlled by foil elements which are also entirely and constantly submerged. I have found that the hydrodynamic eiciency of the craft as measured by its lift over drag ratio is exceptionally high.

In the drawings:

Fig. l is a perspective view of a sport craft supported on water foils constructed according to the principles of this invention and powered by a sail.

Fig. 2 is a view of the craft from ahead, showing the position of the craft in the water when the craft is supported on its displacement hulls.

Fig. 3 is a view of the craft from ahead, showing the position of the craft in the water when the craft is supported on its foils.

Fig. 4 is a plan view of the craft. Fig. 5 is a port side elevation view of the craft.

Fig. 6 is a view of the craft from ahead.

Fig. 7 is a detail of the elevator linkage.

Fig. 8 is a diagrammatic view in fragmentary front elevation of a monoplane or dihedral type of foil embodying the invention.

Fig. 9 is a top plan view of the invention showing control means on the forward and aft foils to control rolling, banking and heeling;

, Fig. 10 is a side elevational view of the modification shown in Fig. 9; and

Fig. l1 is a diagrammatic View in perspective of the modification of Figs. 9 and 10 showing control means for the ailerons on the dihedral supporting foil and the aft foil.

Referring to the drawings, the craft shown is a sailing craft comprising twin displacement hulls 10 secured together by aft beam 9 and forward beam 11 formed as a truss. Two trusses 8 (Fig. 4) join the beam 11 at its midpoint to form a support for the mast 12 on which'is mounted a sail 13 with rigging 14.` Rigidly attached inside each of the twin hulls 10 near its mid-section and extending downwardly therefrom are forward struts 15 of low resistance fastened at the bottom to a dihedral hydrofoil 16 of high aspect ratio and low profile drag. Beams 9 and 11, both extending athwartship, provide the rigidity necessary for the load-carrying portion of the craft to withstand the upward bending moments developed by the lifting forces transferred to it from foil 16 through struts 15 and strength members 10a joining the beams. Attached at the rear of the twin hulls 10 and extending downwardly therefrom are struts 17 of low resistance fastened at the bottom to the ends of a hydrofol 18 having rotatably mounted on its trailing edge an elevator flap 19. Rigidly connected to elevator flap 19 is a lever 20 (Fig. 7) extending into port rear strut 17 and pivoted to rod 21. Rod 21 is pivoted to bell crank 22 rotated` about pin 23 by rod 24 which is actuated by lever 25 (Fig. l). Rotatably mounted on the after side of port aft strut 17 is steering flap 26 which is firmly attached at the top to the middle of bar 27 (Fig. 4) so that lthe bar is perpendicular to the plane of steering ap 26. Taut guy wires 28` extend from the ends of bar 27 to points near 2 the corresponding ends of control bar 29 pivoted in the middle for horizontal turning about pin 30, and so constructed that the operator in seat 31 canconveniently turn the control bar 29 with his feet and thus rotate steering Before the craft gains enough speedto become foil supported, operation of the embodiment shown in the drawing is much like operation of conventional catamaran sailing craft. Steering is achieved by turning the steering flap 26 with control 29. Forward struts 15 and aft struts 17 perform the function of sideboards or a keel in reducing leeward slip. The twin hulls 10 are designed for low resistance when afloat so that the craft can readily accelerate enough to become dynamically supported on its foils.

When the craft is at rest, the twin hulls 10 support it by displacement so that the water line is as shown in Fig. 2. Foil lift is nearly proportional to the square of velocity. Thus lift increases rapidly as craft forward velocity iricreases. Upon starting, there is some foil lift which raises the hulls 10 above the level of Fig. 2. At still higher speed, the foil lift is sufficient to raise the entire weight of craft and crew. The water line is then below the bottom of the displacement hulls 10 as in Fig. 3. Hulls 10 are shaped with downwardly converging side walls for gradual emergence without discontinuity as foil lift increases. Elevator ilap 19 may beV operated by control 25 to vary lift coefiicient and stabilize foil operating depth for existing wind and load conditions.

For greater stability and consequently less need for adjustment of elevator ap 19, the craft has its center of gravity almost vertically above the trailing edge of the forward foil 16. See Figures 4 and 5 wherein the center of gravity is illustrated as along the line CG-CG. Thus most of the weight is supported by the forward foil 16, while the rear foil 18 exerts only slight positive or negative lift to maintain the desired operating depth of the main foil under various conditions. Downward rotation of the elevator tlap 19 increases lift on the rear foil 18 and thus tends to raise the rear of the craft relative to the front. This movement lowers the lift coecient of the main lifting foil 16 and that foil tends to move deeper in the water. Conversely, upward rotation of elevator iiap 19 tends to move the main lifting foil 16 nearer to the surface of the water.

Consideration of hydrofol craft with sails as a means of propulsion requires special attention to pitching moments and heeling moments `from the sail. Pitching moments are those tending to change the elevation of the bow relative to the stern while heeling moments are those teilding to change the elevation of one side relative to the ot er.

Lestfoil depth be varied by changing sail pitching moment from change in wind or tack, the elevator ap 19 must be operated to counteract such fluctuations in pitching moment. Upward flap rotation produces a moment to counteract an increase in sail pitching moment which would otherwise depressthe bow. Downward -ap rotation produces a moment to counteract a reduction in sail pitching moment. Inclusion of some means to perform this function is desirable for sail propelled hydrofol craft in order to achieve stable foil operating depth.

The purpose of the dihedral angle in the forward foil 16 is to counteract heeling moments. As seen in Figure 8, the dihedral foil 16 has interconnected identical wing elements 16a and 16b, their centers of lift being designated respectively L and L.- If normals N and N are projected upwardly respectively from wing elements 16a and 16b at L and L', their point of intersection P will be materially above the center of gravity designated G for the craft. The craft center of gravity is below the point of intersection of the normals to the centers of lift of both halves of the dihedral. When the craft heels, there is some lateral craft slip despite the presence of struts. As a result of this sideways movement, lift forces are developed on the dihedral foil in addition to the lift resulting from trie forward motion of the craft. The initial reaction lift from the sideways movement of thedihedral, however, is an upward lift on the leeward half of the dihedral, and a downward or negative lift on the Windward half. A

theleeward side of the craft relative to the Windward side. Thus stability is achieved without resort to wings or other stabilizing surfaces in air. As a modification, aileron fiaps may be used on one or more of the foils to control rolling, banking and heeling.

Although the main foil is completely submerged during normal operation to avoid excessive drag, sudden strong gusts of wind may bring the Windward tip of the foil to the surface and thus reduce lift on the Windward side. Thereby the main foil has a powerful stabilizing action under extreme operating conditions. Since the foil has a dihedral angle, the tip emerges while the craft is yet at a lesser angle of heel than would be the case with a flat foil using struts of the length shown.

Craft embodying my invention preferably has a high lift to drag ratio and thereby high hydrodynamic efficiency. This advantage is achieved by a high aspect ratio of the supporting foil and by the substantially constant submergence of both the high aspect ratio supporting foil and the stabilizing surface. Aspect ratio is the ratio of the square of foil span to the foil supporting area and high aspect ratio is achieved by having foil length many times greater than foil chord. When lift is exerted on a foil, the pressure on the lower surface of the foil exceeds the pressure on the upper surface. The water has a tendency, therefore, to flow around the foil tips. This tip flow results in an energy loss referred to as induced drag. The amount of induced drag per unit of lift is almost inversely proportional to the aspect ratio so that as the span is increased in length, the tip loss becomes less significant. With the high aspect ratios achieved by my invention, induced drag losses are kept very small.

The foil 16 not only has a high aspect ratio, but it is also thin for the material employed, so that the foil also has low profile drag. The foil illustrated is of wood and it would be thinner if made of metal.

Such a thin foil of high aspect ratio is possible because in my combination, it is not necessary that the lifting foil incorporate structural elements to withstand severe bending moments. A considerable portion of the structural mass necessary for foil rigidity, instead of being incorporated in the submerged foil, is transferred to the beams 9 and 11 carried in air. The forces on the lifting foil are transferred by the struts to this structure in air. The lifting foil, therefore, need only withstand the comparatively small bending moments developed between the struts. The drag introduced by having this additional structure in air is not of significance compared to the decrease in drag from using the thin lifting foil of high aspect ratio.

As best shown in Figs. 9 and 10, a plurality of ailerons may be carried by the dihedral foil 16 to enhance roll control. Each aileron 32 is hingedly mounted upon the dihedral foil 16 at the trailing edge thereof and preferably at the lateral extremities of the dihedral. An aileron horn 33, disposed within the strut 15, is secured to the aileron 32. Linkage means includes a rod 34 extending vertically through the strut and hull structure to engage a bell crank 35 mounted for rotation upon the hull structure. A torque tube fixedly engages the bell crank and is rotatably mounted upon the hull 10 as at 36. The control stick for the ailerons is substantially the same as that which has been shown in Figs. l, 4 and 5. This control stick however, is adapted to universal movement arcuately fore and aft of the torque tube and also laterally thereof. A bell crank similar to that shown in Fig. 10 is disposed on the starboard hull as at 37 of Fig. 9. Bell cranks 35 and 37 of both hulls are interconnected through connecting rod 38. The connecting rod running athwartship actuates, in unison with the port bell crank, the bell crank on the starboard side of the craft which in turn through a vertical rod moves the aileron horn and aileron disposed on the starboard of the dihedral foil. It is preferred that the respective ailerons be counter-rotatable.

To further enhance the control of rolling, banking and heeling moments upon the craft, aileron means 39, 40 are are disposed aft of the foil 18. Thus referring to Fig. 9, it will be seen that the foil 18 may comprise a relatively fixed section, and the elevator ap 19, flanked by two or more control sections such as ailerons 39 and 40.

The torque bar 44 axially mounts control stick 25 and extends aft, said torque bar being connected to crank 41 and pivoted at 42 to permit sideways motion of control stick 25. Rods 34 and 43 extend down through struts and 17 respectively and connect with aileron horns 33 mounted upon ailerons 32 of foil 16 and levers 45 mounted upon ailerons 39 and 40 of foil 18 diagrammatically shown in Figure ll. These rods serve to simultaneously differentially operate ailerons 39 and 40 in conjunction with ailerons 32. Rod 24 serves to actuate elevator flap 19 upon fore and aft movement of control stick 25. Rod 24 connects to bell crank 22 which crank translates motion to rod 45 and through lever 46, torque bar 47 and pulleys 48 and 49 movement of the aileron 19 is effected. Belt means 50 may be employed to connect pulleys 48 and 49. My experiments have shown that fo'r high efficiency the aspect ratio of the supporting foil should be at least 10 and preferably greater.

Although I have shown sail craft, my invention can as well be embodied in craft which is powered by motor.

l claim:

l. Foil supported watercraft of high lift to drag ratio having water foil means, the total foil surface area of which is adapted to remain substantially entirely submerged at. all times and all speeds, means for changing lift coetiicient to accommodate the said substantially constant total submerged foil area to different craft speeds for substantially constant depth of foil submergence at different speeds, the said foil means including at least one supporting foil having its trailing edge disposed adjacent a vertical plane of the center of gravity of the craft and having span so much greater than its average chord that the aspect ratio of the foil is at least approximately l0 so that the proportion of induced drag to total drag from the foil is lessened.

2. Foil supported watercraft of high lift to drag ratio having hull means supported out of the water by water foil means having a total surface area which is adapted to remain substantially entirely submerged at all times and at all speeds, means for changing lift coeicient to accommodate the said substantially constant total submerged foil area to different craft speeds for substantially constant depth of foil submergence at different speeds, the said foil means including at least one supporting foil extending athwartship beneath the hull means having its trailing edge disposed adjacent a vertical plane of the center of gravity of the craft, a plurality of struts extending upwardly from the foil and arranged in spaced relation athwartship to support the said foil at a plurality of points, truss beams incorporated athwartship in the hull means for resisting upward bending, said truss beams extending a distance abeam the hull means in the direction of the foil span the said foil having a span so much greater than its average chord that the aspect ratio of the foil is at least approximately l() so that the proportion of induced drag to total drag from the foil is lessened and being connected to the truss means through the upwardly extending struts so that the foil is supported by the truss means against bending moments resulting from dynamic lift on the said foil.

3. Foil supported watercraft of high lift to drag ratio having hull means supported out of the water by water foil means, the said foil means including at least one supporting foil of high aspect ratio extending athwartship beneath the hull means and having its trailing edge disposed adjacent a vertical Plane of the center of gravity of the craft, a single truss member for the foil incorporated in and extending a distance athwartship of the hull means for resisting upward bending, the said foil having a span comparable with or greater than the width of the said truss member, and a plurality of upwardly extendlng struts connecting the said foil with the said truss member so that the foil is supported by the truss member against bending moments resulting from dynamic lift on the said foil.

4. In foil supported watercraft, hull means supported out of the water, constantly submerged hydrofoil means of hlgh aspect ratio, having its trailing edge disposed adjacent a vertical plane of the center of gravity of the craft, connected to the hull by struts which are shaped in foil profile form for reduced resistance, at least one of the struts ofthe craft having a vertically extending flap integral with the said one strut at its rear portion, and mechanism for moving the tiap to steer the craft.

5. Watercraft including in combination sail means for propulsion of the craft, dihedral water foil means, having its trailing edge disposed adjacent a vertical plane of the center of gravity of the craft, extending athwart for dynamic support of the body of the craft out of the water to lessen wave impact against the body while sailing and a submerged foil element of adjustable lift coefficient for changingv the lift of the water foil means to counteract 5 variations in pitching moments resulting from variations in air pressure on the sail means, said craft having its center of gravity intermediate the point of intersection of the normals to both halves of the dihedral at their center of lift and a point on the dihedral foil means.

6. Watercraft including in combination sail means for propulsion of the craft, water foil means extending athwartship for dynamic support of the body of the craft out of the Water to lessen wave impact against the body while sailing, the said water foil means including a foil, having its trailing edge disposed adjacent a vertical plane of the center of gravity of the craft, having opposed tips and formed with a dihedral angle, and struts spaced athwart to hold said foil disposed at a depth of submergence such that the entire foil, including its tips, is normally .constantly submerged but with the tips in proximity to the water surface so that sudden heeling from a gust of wind on the sail means will cause emergence of the'foil tip on the Windward side, while the leeward tip remains submerged, the center of gravity of the craft being intermediate the point of intersection of the normals to both halves of the dihedral at their centers of lift and a point on the foi] means so that heeling moments on the craft are automatically, at least partially, counteracted.

7. Hydrofoil craft of predetermined overall beam having a plurality of hulls disposed side by side for high stability in roll when the craft is supported by displacement, at least one strut extending downward from each hull, and a dihedral foil held constantly submerged by said struts extending athwartship with a span at least as great as the overall beam of said hull means for high stability in roll and low induced drag when the craft is foil supported, said foil having its trailing edge disposed immediately forward of a vertical plane of the center of gravity of the craft.

8. Watercraft having a predetermined center of gravity including in combination sail means for propulsion of the craft, water foil means of high aspect ratio for dynamic support of the body of the craft out of the Water to lessen wave impact against the body while sailing, said water foil means having a trailing edge and secured to said water craft with its trailing edge substantially in vertical alignment with said center of gravity and a submerged foil element of adjustable lift coeicient for changing the lift of the water foil means to counteract variations in pitching moments resulting from variations in air pressure on the sail means.

9. Foil supported water craft of high lift to drag ratio having a predetermined center of gravity and water foil means, the total foil surface area of which is adapted to remain substantially entirely submerged at all times and all speeds, said water foil means including at least one supporting dihedral foil with span so much greater than its average chord that the aspect ratio of the foil is at least approximately 10, so that the proportion of induced drag to total drag from the foil is lessened, said dihedral foil having a trailing edge and being secured on said water craft with its trailing edge substantially in vertical alignment with said center of gravity, and said water foil means also including at least one stabilizing foil for changing the lift coefficient of the supporting dihedral foil.

10. In Water craft of the type having a load carrying structure dynamically supported in air by constantly submerged hydrofoil means, said water craft having a predetermined center of gravity, the said hydrofoil means including at least one foil having a span comparable with or greater than the width of the load carrying structure supported in air and having a ratio of foil span to foil chord of 10 to l or higher, said foil having a trailing edge and secured to said water craft with its trailing edge substantially in vertical alignment with said center of gravity.

ll. Foil supported water craft of high lift to drag ratio having water foil means, a total foil surface area of which is adapted to remain substantially entirely submerged at all times and all speeds, means for changing lift coetiicient to accommodate the said substantially constant total submerged foil area to different craft speeds for substantially constant depth of foil submergence at different speeds, the said foil means including at least one supporting foil with span so much greater than its average chord that the aspect ratio of the foil is at least approximately l so that the proportion of induced drag from the foil is lessened, said one supporting foil being disposed so that 6 a trailing edge thereof transversely intersects the vertical plane of the center of gravity of the craft on at least one point of the trailing edge of the foil.

l2. Watercraft of the type defined in claim 2 in which said one supporting foil carries ailerons for controlling heeling moments on the craft.

13. Watercraft of the type defined in claim 3 including aileron means on said one supporting foil to control heeling moments on the craft.

14. Watercraft of the type defined in claim 4 in which said constantly submerged foil mounts aileron means to control heeling moments on the craft.

l5. Watercraft of the type defined in claim 5 in which both said dihedral water foil means and said submerged foil element of adjustable lift coefficient mount aileron means to control heeling and rolling moments upon the cra t.

16. Watercraft of the type defined in claim 6 in which said dihedral water foil mounts aileron means for controlling heeling and rolling moments upon the craft.

17. Watercraft of the type defined in claim 7 in which said one strut mounts a vertically extending flap, integral and coextensive therewith at its rear portion, and mechanism for moving the flap to steer the craft.

18. Watercraft of the type defined in claim 17 in which said dihedral foil mounts aileron means therewith to control heeling moments upon the craft.

19. Watercraft of the type defined in claim 18 further comprising a submerged foil and means of adjusting the lift coefficient thereof for changing the angle and lift of the dihedral foil to counteract variations in pitching moments resulting from vertical accelerations caused by the orbital motion of the Seaway, and aileron means mounted upon said dihedral foil and said submerged foil of adjustable lift coefficient, to control heeling moments upon the craft.

20. Watercraft of the type defined in claim 19, further comprising truss means for the dihedral foil incorporated in and extending a distance athwartship of the hulls in the direction of the foil span for resisting upward bending, and in which said foil has a span so much greater than its average chord that the aspect ratio of the foil is at least approximately l0.

2l. Foil supported watercraft of high lift to drag ratio having water foil means, the total foil surface area of which is adapted to remain substantially entirely submerged at all times and all speeds, means for changing lift coeicient to accommodate the said substantially constant total submerged foil area to different craft speeds for substantially constant depth of foil submergence at different speeds, the said foil means including at least one supporting foil having its trailing edge disposed immediately contiguous a vertical plane of the center of gravity of the craft and having span so much greater than its average chord that the aspect ratio of the foil is at least approximately l0 so that the proportion of induced drag to total drag from the foil is lessened.

22, Foil supported watercraft of high lift to drag ratio having hull means supported out of the water by water foil means, the said foil means including at least one supporting foil of high aspect ratio extending athwartship beneath the hull means, having its trailing edge disposed adjacent a vertical plane of the center of gravity of the craft, a single truss member for the foil incorporated in and extending a distance athwartship of the hull means for resisting upward bending, the said foil having a span comparable with or greater than the width of the said truss member, and a plurality of upwardly extending struts connecting the said foil with the said truss member so that the foil is supported by the truss member against bending moments resulting from dynamic lift on the said foil.

23. Hydrofoil craft of predetermined overall beam having a plurality of hulls disposed side by side for high stability in roll when the craft is supported by displacement, at least one strut extending downward from each hull, and a dihedral foil held constantly submerged by said struts extending athwartship with a span at least as great as the overall beam of said hull means for high stability in roll and low induced drag when the craft is foil supported, said foil having its trailing edge disposed immediately forward of a vertical plane of the center of gravity of the craft, truss beams extending athwartship the hulls for resisting upward bending, said truss beams being coextensive with the distance between the said hulls,

said foil being connected to said truss beams through said struts so that the foil is supported by the truss beams against bending moments resulting from dynamic lift on the said foil.

24. Foil supported water craft of high lit't to drag ratio having water foil means, the total foil surface area of which is adapted to remain substantially entirely submerged at all times and all speeds, said water foil means including at least one supporting dihedral foil with span so much greater than its average chord that the aspect ratio of the foil is at least approximately ten so that the proportion of induced drag to total drag from the foil is lessened, and said water foil means also including at least one stabilizing foil for changing the lift coeflicient of the supporting dihedral foil, aileron means mounted upon both the supporting dihedral foil and the stabilizing foil to augment control of heeling and rolling moments upon the craft, the center of gravity of said water craft being intermediate the point of intersection of the normals to both halves of the dihedral foil at their centers of lift and a point on the dihedral foil.

References Cited in the tile of this patent UNITED STATES PATENTS 

